Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic projection apparatus includes, a radiation source for providing a projection beam of radiation, a support structure for supporting a patterning device, the patterning device serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate, a projection system for projecting the patterned beam onto a target portion of the substrate, the radiation source further includes, an illumination system for conditioning the beam of radiation so as to provide a conditioned radiation beam so as to be able to illuminate the patterning device; the illumination system defining a plane of entrance wherein the radiation beam enters the illumination system, and a beam delivery system comprising redirecting elements for redirecting and delivering the projection beam from a radiation source to the illumination system. The beam delivery system includes an imaging system for imaging the radiation beam from an object plane located at a distance from the plane of entrance to an image plane located near or at the plane of entrance. In this way the influence of laser pointing drift on both beam position and pointing drift at the entrance is highly decreased.

[0001] This application claims priority to European Patent ApplicationNo. 03076142.3, filed Apr. 17, 2003, herein incorporated by reference inits entirety.

FIELD OF THE INVENTION

[0002] The present invention relates generally to a lithographicprojection apparatus and more particularly to a lithographic projectionapparatus having a beam delivery system for delivery radiation from asource to an illumination system.

BACKGROUND OF THE INVENTION

[0003] The term “patterning device” as here employed should be broadlyinterpreted as referring to means that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate; theterm “light valve” can also be used in this context. Generally, the saidpattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such patterning device include:

[0004] A mask. The concept of a mask is well known in lithography, andit includes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired;

[0005] A programmable mirror array. One example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, the saidundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-adressablesurface. An alternative embodiment of a programmable mirror arrayemploys a matrix arrangement of tiny mirrors, each of which can beindividually tilted about an axis by applying a suitable localizedelectric field, or by employing piezoelectric actuation means. Onceagain, the mirrors are matrix-addressable, such that addressed mirrorswill reflect an incoming radiation beam in a different direction tounaddressed mirrors; in this manner, the reflected beam is patternedaccording to the addressing pattern of the matrix-adressable mirrors.The required matrix addressing can be performed using suitableelectronic means. In both of the situations described hereabove, thepatterning device can comprise one or more programmable mirror arrays.More information on mirror arrays as here referred to can be gleaned,for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patentapplications WO 98/38597 and WO 98/33096, which are incorporated hereinby reference. In the case of a programmable mirror array, the saidsupport structure may be embodied as a frame or table, for example,which may be fixed or movable as required; and

[0006] A programmable LCD array. An example of such a construction isgiven in U.S. Pat. No. 5,229,872, which is incorporated herein byreference. As above, the support structure in this case may be embodiedas a frame or table, for example, which may be fixed or movable asrequired.

[0007] For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

[0008] Lithographic projection apparatus can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, thepatterning device may generate a circuit pattern corresponding to anindividual layer of the IC, and this pattern can be imaged onto a targetportion (e.g., comprising one or more dies) on a substrate (siliconwafer) that has been coated with a layer of radiation-sensitive material(resist). In general, a single wafer will contain a whole network ofadjacent target portions that are successively irradiated via theprojection system, one at a time. In current apparatus, employingpatterning by a mask on a mask table, a distinction can be made betweentwo different types of machine. In one type of lithographic projectionapparatus, each target portion is irradiated by exposing the entire maskpattern onto the target portion in one go; such an apparatus is commonlyreferred to as a wafer stepper or step-and-repeat apparatus. In analternative apparatus—commonly referred to as a step-and-scanapparatus—each target portion is irradiated by progressively scanningthe mask pattern under the projection beam in a given referencedirection (the “scanning” direction) while synchronously scanning thesubstrate table parallel or anti-parallel to this direction; since, ingeneral, the projection system will have a magnification factor M(generally<1), the speed V at which the substrate table is scanned willbe a factor M times that at which the mask table is scanned. Moreinformation with regard to lithographic devices as here described can begleaned, for example, from U.S. Pat. No. 6,046,792, incorporated hereinby reference.

[0009] In a manufacturing process using a lithographic projectionapparatus, a pattern (e.g., in a mask) is imaged onto a substrate thatis at least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g., anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

[0010] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation source may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam, and such components may also bereferred to below, collectively or singularly, as a “lens”. Further, thelithographic apparatus may be of a type having two or more substratetables (and/or two or more mask tables). In such “multiple stage”devices the additional tables may be used in parallel, or preparatorysteps may be carried out on one or more tables while one or more othertables are being used for exposures. Dual stage lithographic apparatusare described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791,both incorporated herein by reference.

[0011] Although specific reference may be made in this text to the useof the apparatus according to the invention in the manufacture of ICs,it should be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid crystal display panels,thin film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

[0012] In the present document, the terms “radiation” and “beam” areused to encompass all types of electromagnetic radiation, includingultraviolet (UV) radiation (e.g., with a wavelength of 365, 248, 193,157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having awavelength in the range 5-20 nm), as well as particle beams, such as ionbeams or electron beams.

[0013] Prior art photolithographic systems generally comprise aradiation source located at a certain distance from the actualprojection system. In most setups, the generators of radiation, such aslaser systems or the like, are situated in another location, oftentimeseven on a different floor in a building. A regular setup for such alithographic system is a laser that is located on one floor of abuilding, and an illumination and projection system that is located onanother floor. Small or large, such a geometrical distance introducesthe need for a beam delivery system for transporting and delivering thebeam from the radiation source to the location where the incidentradiation is actually used in the photolithographic process, i.e., to anillumination system wherein the radiation beam is first conditioned soas to provide optimal illumination characteristics, and then projectingthe beam in order to transfer a lithographic pattern from a patterningdevice, such as a mask, to a target material. The prior art beamdelivery systems generally comprise a set of reflective mirrors, fromwhich generally at least two mirrors are tiltable, one mirror located inthe vicinity of the illumination system and one mirror located at adistance of the illumination system. An illustrative setup for suchprior art beam control is for instance depicted in FIG. 2.

[0014] Obviously, a reflection of a radiation beam in these tiltablemirrors (also referred to as steering mirrors) affect both the pointingdirection of the beam and the position where the beam enters theillumination system. Specifically, a beam reflected by a tiltable mirrorlocated at a relatively large distance from such illumination system,will, upon tilt thereof enter at a displaced position into theillumination system, where the deviation in pointing direction of thebeam will remain relatively small. In contrast, a beam reflected by atiltable mirror located at a relatively small distance from suchillumination system, will, upon rotation thereof enter at an almostunaltered location, where the pointing of the beam will vary quitedramatically. Thus, by tilting both steering mirrors, a controlledpointing direction and entrance position of the beam is possible, whereboth parameters can be tuned to an optimal setting.

[0015] These parameters are usually controlled relative to a plane ofentrance wherein said radiation beam enters said illumination system, sothat in a working photolithographic setup, exact requirements can be metas to inter alia the entering position of the radiation beam and thepointing of said beam at or near said plane of entrance. Thesecharacteristics can allow the provision of a radiation beam that offersoptimal illumination characteristics for attaining the desiredminuteness of patterns that are produced in current systems.

[0016] However, such a coupling of pointing and position control by thesteering mirror systems in the prior art photolithographic systemsrequires a relatively complicated correction method, which has to takeinto account that one correction in one degree of freedom induces achange of the radiation beam in another degree of freedom. Furthermore,radiation sources generally suffer from intrinsic time variations inpositional and directional output of radiation, which is for instancecaused by the laser processes in the source itself. A drift in forinstance laser pointing causes a relatively large drift in beamposition. These variations also should be detected and controlled, forexample, using closed-loop control. Because the timeframe for thesevariations can range from picoseconds to seconds or even more, a hugebandwidth is needed to control all errors. Therefore, it is very hard tohave a robust closed-loop correction for these effects. Hence, theinventors have identified a need to provide a lithographic projectionapparatus which is less sensitive to such coupling of direction andpositional alignment variations and radiation source imperfections.

[0017] Furthermore, it has been found that in certain systems a changein position, i.e., a translation of the beam in transversal direction,induces a relative large impact on relevant illumination parameters suchas uniformity and angular distribution of the beam on reticle level.Therefore it is desired to obtain a maximum control on the entranceposition of the beam.

SUMMARY OF THE INVENTION

[0018] Hence, embodiments of the present invention provide alithographic system wherein variations of the radiation source have aless serious effect on these parameters and wherein a coupling betweenthe position and pointing parameters of the radiation beam parameters isultimately absent.

[0019] In accordance with embodiments of the invention, aphotolithographic system includes a beam delivery system thatincorporates an imaging system for imaging said radiation beam from anobject plane located at a distance from said plane of entrance to animage plane located near or at said plane of entrance.

[0020] In this way, a translational effect by a variation in thepointing direction of the beam, even located at a certain physicaldistance of the illumination system, is annihilated by the imagingsystem, which effectively transfers and images pointing and positionproperties of the beam from the object plane to the image plane.Therefore, a tilt of the beam in the object plane does not lead to atranslation near the plane of entrance, since the object plane is imagedand no translation occurs. By a suitable combination of imaging systemparameters obviously the distance of the object to image plane can becontrolled and the magnification of the object beam. In one embodiment,said imaging system is a 1× imaging system.

[0021] An embodiment of the imaging system includes a pair of lenses,each lens having a focal distance of ¼ times said distance from saidobject plane to said image plane. Such an imaging system leads to amagnification of one. By such an imaging system arrangement, a completeuncoupling of pointing direction and position of the beam is possible aswill be explained below.

[0022] By the photolithographic system according to the invention acomplete uncoupling of tilt and translation occurs when a tiltablemirror is located in an object plane of said imaging system. It is thenpossible to control the position of the beam by translatable mirrors,that is, in a system wherein said beam delivery system comprises atleast one translatable mirror for translating said projection beam in aat least one direction transverse to a beam direction.

[0023] Although the pointing direction of said beam is controllable by aplurality of tiltable mirrors each controlling a rotation about adifferent rotation axis, in another configuration, each mirror istiltable in two different directions. Furthermore, said mirror may betranslatable, in order to control at least one direction of movement ofthe beam for controlling the position of said beam.

[0024] The invention also relates to a device manufacturing methodincluding, providing a substrate that is at least partially covered by alayer of radiation-sensitive material, providing a projection beam ofradiation using a radiation source, delivering said projection beam fromsaid radiation source to said illumination system, conditioning saidprojection beam using an illumination system, said illumination systemdefining a plane of entrance wherein said radiation beam enters saidillumination system, using a patterning device to endow the conditionedprojection beam with a pattern in its cross-section, and projecting thepatterned beam of radiation onto a target portion of the layer ofradiation-sensitive material. According to embodiment of the invention,the method includes imaging said radiation beam from an object planelocated at a distance from said plane of entrance to an image planelocated near or at said plane of entrance. In one embodiment, the methodincludes locating a tiltable mirror in an object plane of said radiationbeam so as to control a pointing direction of the beam at the plane ofentrance. The method further includes reflecting the radiation beam bytranslatable mirrors, so as to control the beam position at the plane ofentrance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich corresponding reference symbols indicate corresponding parts, andin which:

[0026]FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

[0027]FIG. 2 depicts a beam delivery system according to the prior art;

[0028]FIG. 3 depicts a beam delivery system according to the invention;

[0029]FIG. 4 depicts a detailed view of imaging a tilted light beam froman object plane to an image plane;

[0030]FIG. 5 depicts a schematic perspective view of a first mirrorconfiguration showing translatable mirrors;

[0031]FIG. 6 depicts a schematic perspective view of a second mirrorconfiguration showing translatable mirrors.

DETAILED DESCRIPTION

[0032] Embodiment 1

[0033]FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

[0034] a radiation source Ex, BD, IL, for supplying a projection beam PBof radiation (e.g., light in the deep ultraviolet region). In thisparticular case, the radiation source also comprises a radiation sourceLA;

[0035] a first object table (mask table) MT provided with a mask holderfor holding a mask MA (e.g., a reticle), and connected to firstpositioning means PM for accurately positioning the mask with respect toitem PL;

[0036] a second object table (substrate table) WT provided with asubstrate holder for holding a substrate W (e.g., a resistcoated siliconwafer), and connected to second positioning means PW for accuratelypositioning the substrate with respect to item PL; and

[0037] a projection system (“lens”) PL for imaging an irradiated portionof the mask MA onto a target portion C (e.g., comprising one or moredies) of the substrate W.

[0038] As here depicted, the apparatus is of a reflective type (i.e.,has a reflective mask). However, in general, it may also be of atransmissive type, for example (with a transmissive mask).Alternatively, the apparatus may employ another kind of patterningdevice, such as a programmable mirror array of a type as referred toabove.

[0039] The source LA (e.g., an excimer laser source) produces a beam ofradiation. This beam is fed into an illumination system (illuminator)IL, either directly or after having traversed conditioning means, suchas a beam expander Ex, for example and a beam delivery system BD asshown in FIG. 1. The illuminator IL may comprise adjustable members AMfor setting the outer and/or inner radial extent (commonly referred toas sigma-outer and sigma-inner, respectively) of the intensitydistribution in the beam. In addition, it will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the beam PB impinging on the mask MA has a desireduniformity and intensity distribution in its cross-section.

[0040] It should be noted with regard to FIG. 1 that the source LA maybe within the housing of the lithographic projection apparatus (as isoften the case when the source LA is a mercury lamp, for example), butthat it may also be remote from the lithographic projection apparatus,the radiation beam which it produces being led into the apparatus (e.g.,with said beam delivery system); this latter scenario is often the casewhen the source LA is an excimer laser. The current invention and claimsencompass both of these scenarios.

[0041] The beam PB subsequently intercepts the mask MA, which is held ona mask table MT. Having traversed the mask MA, the beam PB passesthrough the lens PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the second positioning means PW (andinterferometric measuring means IF), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the beam PB. Similarly, the first positioning means PM can beused to accurately position the mask MA with respect to the path of thebeam PB, e.g., after mechanical retrieval of the mask MA from a masklibrary, or during a scan. In general, movement of the object tables MT,WT will be realized with the aid of a long-stroke module (coarsepositioning) and a short-stroke module (fine positioning), which are notexplicitly depicted in FIG. 1. However, in the case of a wafer stepper(as opposed to a step-and-scan apparatus) the mask table MT may just beconnected to a short stroke actuator, or may be fixed. Mask MA andsubstrate W may be aligned using mask alignment marks M1, M2 andsubstrate alignment marks P1, P2.

[0042] The depicted apparatus can be used in two different modes:

[0043] 1. In step mode, the mask table MT is kept essentiallystationary, and an entire mask image is projected in one go (i.e., asingle “flash”) onto a target portion C. The substrate table WT is thenshifted in the x and/or y directions so that a different target portionC can be irradiated by the beam PB; and

[0044] 2. In scan mode, essentially the same scenario applies, exceptthat a given target portion C is not exposed in a single “flash”.Instead, the mask table MT is movable in a given direction (theso-called “scan direction”, e.g., the y direction) with a speed v, sothat the projection beam PB is caused to scan over a mask image;concurrently, the substrate table WT is simultaneously moved in the sameor opposite direction at a speed V=Mv, in which M is the magnificationof the lens PL (typically, M=¼ or ⅕). In this manner, a relatively largetarget portion C can be exposed, without having to compromise onresolution.

[0045]FIG. 2 shows a typical prior art setup for a beam delivery systemBD, wherein the radiation source 2 generates a laser beam 3 that isguided via multiple mirrors and other light guiding elements 4 to aplane of entrance 5 in the illuminator system depicted as IL in FIG. 1.

[0046] In fact, said plane of entrance is a quite mathematicalconstruct; any plane may form a plane of entrance, even a plane locatedat a certain distance from the illumination system or a plane located inor even after the illumination system. Such a plane of entrance mayserve as a reference plane defining a zero-base plane from which thebeam is further guided and conditioned while keeping close control onrelevant imaging parameters. Hence for the purpose of the invention,such plane is generally referred to as “plane of entrance”, although theplane may be located at various other positions.

[0047] In a general setup and as one embodiment, said plane of entranceis usually coincident or nearly coincident with a different opticalelement (DOE), which is an optical element that is usually part of theillumination system IL in FIG. 1. The present invention may also be usedwith illumination systems using optics not incorporating a DOE. Close tothis DOE-level, a beam measuring system (not illustrated) measures interalia the beam position and pointing direction thereof. This informationis required to correct the mirrors 7 and 8 on a regular basis. Incurrent systems, such a correction is done in between subsequentexposure of two wafers. In addition, the laser drift in pointing andposition may be estimated and incorporated in the control of steeringmirrors 2.

[0048] In FIG. 2, a dashed line indicates the presence of a “beamrotation break” 6, which means, that the section before said break,i.e., the section from the laser 2 to the break 6, may be oriented in adifferent direction than the section from the break 6 up to the plane ofentrance 5. This direction usually introduces a 90° difference in bothpolarization and laser beam 3 direction.

[0049] The steering mirrors of FIG. 1 are comprised of a “positioning”steering mirror 7 and a “pointing” steering mirror 8 depicted in FIG. 2.Both mirrors are in this example illustrated as mirrors than arerotatable in two directions (as indicated by the curved arrow sign),thereby providing a tilt in two transversal directions relative to thebeam direction. Obviously, a rotation or tilt of the positioning mirror7 introduces a translation of the beam 3 at the plane of entrance 5,conversely, a rotation of the pointing mirror 8 introduces a change inthe pointing direction of the beam near or at the plane of entrance 5.Evidently, the positioning mirror also introduces a slight change in thepointing direction of the beam and the pointing mirror introduces aslight translation of the beam, hence the translational and pointingeffects of the mirrors 7 and 8 are coupled.

[0050] It has been found that a predominant factor to the uniformity andangular distribution budget on reticle level is due to laser pointingdrift. In this respect, uniformity is related to the spatialdistribution of the radiation at reticle level. However, an unwantedposition shift of the laser beam at the illuminator entrance also leadsto unwanted changes in the angular distribution of the radiation, anddiffers this distribution from the angular distribution of the unshiftedbeam. Hence, small variations Δα in the pointing direction of the laserat a distance away from the DOE-level are magnified by the distance Lbetween the laser and the DOE: a translation of the beam 3 occurs ofmagnitude LΔα. Obviously, such a translation cannot be ignored anddepending on the time scale over which this drift occurs, it is quitedifficult to compensate this effect by closed-loop beam steering setup,where the beam measuring unit measures a deviation and a correctionsignal is generated to control the steering mirrors 7 and 8 in order tocompensate such deviation.

[0051] In the setup of FIG. 3, the geometrical distance remainsunaltered, but the optical distance a laser beam 3 travels from thelaser to the illumination system is reduced by introducing an imagingsystem 9 in the beam delivery path. Although the skilled artisan willappreciate that the imaging system may comprise various alternatives, abasic solution is illustrated by the embodiment depicted in FIG. 3. Thisembodiment comprises two positive lenses introducing an object to imagedistance F and a magnification factor M. Thus, the optical distance isnow reduced by distance F of the imaging system and the geometricaldistance is unaltered.

[0052] In the embodiment, there is depicted a laser 2. After the laser2, a beam expander unit 10 is placed for defining an appropriate shapeof the laser beam. In this example, the unit 10 comprises a conventionalbeam expander optical system. After the beam expander unit 10 atranslatable mirror 11 is placed at a 45° angle with the beam direction.Hence, the beam is reflected at an angle of 90° towards the sectionbreak 6. After the break 6 the system is oriented along the Y axis as isindicated by the reference coordinate system.

[0053] The section before the break 6 is oriented in the Z-direction,although for illustrating purposes depicted as lying in the Y direction.

[0054] In this configuration, a translation of mirror 11 in theZ-direction introduces a translation in Z-direction. The laser beam 3,reflects in a second translatable mirror 12, oriented at a 45° anglewith the beam direction, again reflecting the beam at an angle of 90° inthe Y-direction. A small translation of the mirror 12 in the X directionnow introduces a translation in X-direction. Hence a combination oftranslation of both mirrors 11 and 12 moves the beam in a X-Z planegenerally perpendicular to the beam direction Y. Furthermore, the beammay be reflected by a tiltable mirror, which in the embodimentpreferably is the mirror 12 that is also translatable. In this way,small deviations in the direction of the beam may be compensated byrotating the mirror. Off course, when the mirror is not located near theobject plane of the image system, such a rotation will introduce atranslation of the beam as well, as explained with reference to FIG. 2.

[0055] In FIG. 4 a detailed illustration is given of the imagingcharacteristics of a 1× imaging system 9 to be used in an embodiment ofthe invention.

[0056] The imaging system 9 of the embodiment comprises a pair ofidentical positive lenses 13, 14 having focal distance f and spaced at adistance of 2f.

[0057] In this system, an object plane 15 located at a distance f infront of lens 13 is imaged at a distance f after lens 14. Hence, theoptical distance of the beam is reduced by an amount of 4f. It is notedthat the 1× imaging is relatively insensitive to the exact position ofboth lenses, provided that they are separated by a distance 2f.

[0058] The system 9 of FIG. 4 hence transfers pointing and position ofthe light beams in object plane 15 to exact inverted position andpointing direction in image plane 16. Therefore, a change in directionin the object plane does not result in a change in position in the imageplane—effectively uncoupling the pointing and position characteristicsof the laser beam 4. It is noted that a relative small deviation inducedby pointing drift of the laser is now virtually eliminated by thesystem, since such a deviation does only lead to a very small,ultimately absent translation near the plane of entrance. Hence, theneed for correction of pointing deviations is much smaller, since thesedeviations will not introduce a translation if the pointing deviationscan be projected on the object plane of the imaging system 9.

[0059] By introduction of the imaging system 9 having an object to imagedistance of F=4f and magnification M=1, a pointing deviation Δα now onlyintroduces a translation of$\frac{\Delta \quad {\alpha \left( {L - F} \right)}}{M}.$

[0060] It will be appreciated that when the object to image distance isequal to the geometrical distance L, there is no translational deviationnear the entrance plane of the illumination unit. In that case, beamposition—and pointing control can be completely uncoupled. The steeringand control of the beam pointing and position can therefore beconsiderably simplified and position errors due to laser pointing driftcan be strongly attenuated, thereby increasing the stability of bothuniformity and angular distribution of the radiation beam on reticlelevel.

[0061]FIGS. 5 and 6 show two different perspective views of the mirrorconfiguration depicted in FIG. 3 showing translatable mirrors 11 and 12.

[0062] In FIG. 5, a system including such a translatable mirrorconfiguration shows an orthogonal bend, because the input beam (comingfrom a laser source) points in a different direction than the rest ofthe beam delivery system. To arrive at a system that is oriented in agenerally longitudinal direction, FIG. 6 discloses a configuration,wherein a further mirror 17 reflects the beam in a desired longitudinaldirection. Both solutions rotate both the laser beam aspect and itspolarization, which can be rotated back in well known ways if required.

[0063] While specific embodiments of the invention have been describedabove, it will be appreciated that the invention may be practicedotherwise than as described. In the embodiments, the imaging system is a1× imaging system. Other magnifications may be introduced as well, aswell as other beam shaping and conditioning prior to entering theillumination unit. The description is not intended to limit theinvention.

What is claimed is:
 1. A lithographic projection apparatus comprising: asupport structure for supporting a patterning device, the patterningdevice serving to pattern a beam of radiation according to a desiredpattern; a substrate table for holding a substrate; a projection systemfor projecting the patterned beam onto a target portion of thesubstrate; an illumination system for conditioning said beam ofradiation to provide a conditioned radiation beam that illuminates saidpatterning device, said illumination system defining a plane of entrancewherein said radiation beam enters said illumination system; and a beamdelivery system comprising redirecting elements for redirecting anddelivering said beam to said illumination system, wherein said beamdelivery system comprises an imaging system constructed and arranged toimage said radiation beam from an object plane located at a distancefrom said plane of entrance to an image plane located proximate saidplane of entrance.
 2. A lithographic projection apparatus according toclaim 1, wherein said imaging system is a 1× imaging system.
 3. Alithographic projection apparatus according to claim 2, wherein saidimaging system comprises a pair of lenses, each lens of said pair havinga focal distance of ¼ times said distance from said object plane to saidimage plane.
 4. A lithographic projection apparatus according to claim3, wherein each lens of said pair comprises a reflective opticalelement.
 5. A lithographic projection apparatus according to claim 1,wherein said beam delivery system comprises at least one translatablemirror for translating said projection beam in a at least one directiontransverse to a beam direction.
 6. A lithographic projection apparatusaccording to claim 5, wherein two translatable mirrors are positioned insubsequent positions of the beam delivery path, wherein a first mirrortranslates said beam in a first direction and a second beam translatessaid beam in a second direction, said first and second directions beingtransverse to each other and to said beam direction.
 7. A lithographicprojection apparatus according to claim 5, wherein said translatablemirror is placed in said object plane.
 8. A lithographic projectionapparatus according to claim 1, wherein a tiltable mirror is located inan object plane of said imaging system.
 9. A lithographic projectionapparatus according to claim 8, wherein said tiltable mirror isrotatable in two different directions.
 10. A lithographic projectionapparatus according to claim 8, wherein said tiltable mirror istranslatable.
 11. A device manufacturing method comprising: delivering aprojection beam of radiation from a radiation source to an illuminationsystem; conditioning said projection beam using an illumination system,said illumination system defining a plane of entrance wherein saidradiation beam enters said illumination system; patterning theconditioned projection beam with a pattern in its cross-section; andprojecting the patterned beam of radiation onto a target portion of alayer of radiation-sensitive material on a substrate; and imaging saidradiation beam from an object plane located at a distance from saidplane of entrance to an image plane located proximate said plane ofentrance.
 12. A method according to claim 10, wherein the methodcomprises: locating a tiltable mirror in an object plane of saidradiation beam so as to control pointing direction of the beam at theplane of entrance; and reflecting said radiation beam by translatablemirrors, so as to control the beam position at the plane of entrance.